1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with fuel rods in a fuel assembly containing annular nuclear fuel pellets having the same U-235 enrichment and different annulus sizes for graduated enrichment loading.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In nuclear reactors generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. Specifically, in a BWR the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. A hollow water cross extends axially through the outer channel so as to provide an open inner channel for subcooled moderator flow through the fuel assembly and to divide the fuel assembly into four, separate, elongated compartments, each containing a mini-bundle of similar design of the fuel rods. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows through the water cross and along the fuel rods removing energy from their heated surfaces.
Current BWR fuel rod bundle designs make use of solid pellet fuel rods having five to seven U-235 enrichments within a single bundle. These multiple enrichments are needed for radial power shaping within each bundle in order to maintain the fuel rod power peaking acceptably low. However, the requirement of multiple enrichments increases the cost and time required for fabrication. While only a small number of fuel rods in the assembly are involved, approximately one-third of the rods, they require four enrichments to be fabricated. Therefore, just to make these few rods, the UF.sub.6 gas to powder chemical conversion line has to be flushed out four times which is both costly and time-consuming. Market considerations do not justify dedicating a conversion line for each enrichment. The only other alternative, e.g., creating the enrichments needed by blending two or three enrichments, is costly and time-consuming.
Consequently, a need exists for a fresh approach to fuel bundle design which will overcome the problems associated with the requirement for multiple fuel enrichment.